Magnesium and magnesium alloys offer a combination of low specific gravity and excellent strength for applications such as vehicle bodies and components. But, magnesium and magnesium alloys are subject to oxidation and other corrosive reactions in humid environments. Therefore, magnesium-based substrates are often coated with a coating formed from an electrocoat coating composition. Such coatings may be deposited on the substrate via electrodeposition, e.g., electrocoating, to minimize oxidation and corrosive reactions.
Problematically, however, magnesium often dissolves in electrocoat coating compositions, particularly in electrocoat coating compositions having a pH of less than 11. Magnesium-based substrates are particularly at risk for magnesium dissolution during initial immersion in the electrocoat coating composition since the substrate may not yet be cathodic protected. Magnesium dissolution produces corrosion products such as Mg2+, OH−, and H2, which further increases the pH of the electrocoat coating composition.
Additionally, magnesium hydroxide, formed from Mg2+ and OH−, may settle out of the electrocoat coating composition and render the composition unsuitable for continued electrocoating. Such fouling and necessary replacement of the remaining electrocoat coating composition is costly and time-consuming on an industrial scale.
Finally, corrosion products and magnesium hydroxide may also contribute to a reduced quality of the coating disposed on the substrate, which may in turn accelerate oxidation and other corrosive reactions of the substrate under humid conditions.